Why is this a problem? People sell it for profit and when it finally come to a historian or archaeologist they have no way of knowing where it came from and if this could be a significant find. This may be part of a great historic site that has yet to be uncovered and without the provenience of the item, it means nothing. It leaves us in the dark. It maybe neat and fascinating but it could have had a wealth of information it no longer has.
This lose of history more than anything else. Check out more on this subject at "Florida Public Archaeology".